The present invention relates to an apparatus for coding/decoding digital information such as an audio signal and, more particularly, to an apparatus for detecting a transmission channel error using an error detection code.
Generally, to reduce transmission channel errors generated by transmission channel noise, an error detection code is used. However, if the transmission channel noise is large, transmission channel errors cannot be completely removed even when the error detection code is used. Additionally, information decoded from a received code with a transmission channel error may largely degrade. To cope with these situations, after error correction processing using an error-correcting code, a transmission channel error is detected in every unit time length (or bit length) using an error detection code of CRC (Cyclic Redundancy Check) or the like, and information resending is requested or error concealment processing is performed.
Details of an error-correcting code and error detection code are described in H. Imai, xe2x80x9cCode Theoryxe2x80x9d, the Institute of Electronics, Information and Communication Engineers, pp. 118-120, 1990 (reference 1). There is also a method of repeatedly using information obtained in the preceding unit time as error concealment processing. Details of application of this method to audio coding are described in T. Ohya et al., xe2x80x9cPitch Synchronous Innovation CELP (PSI-CELP)xe2x80x9d, technical report of IEICE, RCS93-78, pp. 63-70, November 1993 (reference 2).
On the other hand, there is a communication system which defines the above-described error correction processing or error detection processing not as digital information coding/decoding processing but as part of the transmission system. This communication system is described in xe2x80x9cVolume 3, Specifications of Air-Interface for 3G Mobile Systemxe2x80x9d, Association of Radio Industries and Business (ARIB), p. 118 and Annex D, July 1998 (reference 3). In this communication system, a CRC error detection code is added in every the above-described unit time length. In decoding processing, an error can be detected in every unit time length.
Conventional systems associated with transmission channel error detection will be described with reference to FIGS. 6 and 7.
In a conventional error detection code addition apparatus shown in FIG. 6, a coding circuit 3 encodes digital information input to an input terminal 1, transfers the obtained coded information to a CRC circuit 7, and also outputs the information from an output terminal 9. The CRC circuit 7 calculates an error detection code from the coded information transferred from the coding circuit 3 and outputs the obtained error detection code from an output terminal 11. In the conventional error detection code addition apparatus, processing by each circuit is performed in every coding (or decoding) unit time length.
In a conventional error detection apparatus shown in FIG. 7, an error detection circuit 29 detects a transmission channel error using a detection code input to an input terminal 23 and outputs the detection result to a decoding circuit 27. The decoding circuit 27 decodes digital information from coded information input to an input terminal 21. When the detection result output from the error detection circuit 29 represents error detection, the above-described concealment processing is performed. The decoding circuit 27 outputs the decoded digital information from an output terminal 35. In the conventional error detection apparatus, processing by each circuit is performed in every coding (or decoding) unit time length.
An example of processing by the coding circuit 3 and decoding circuit 27 is an apparatus using an audio coding algorithm described in reference 2. Details of processing by the CRC circuit 7 and error detection circuit 29 are described in reference 1.
As a problem of the above-described prior arts, the transmission channel error detection result cannot be obtained in an arbitrary coding (or decoding) unit time length. This is because the communication system detects a transmission channel error in every predetermined unit time length.
It is an object of the present invention to provide a transmission channel error detection code addition apparatus and error detection apparatus which can obtain a transmission channel error detection result in an arbitrary coding unit time length in which digital information is given.
In order to achieve the above object, according to the present invention, there is provided a transmission channel error detection code addition apparatus comprising division means for dividing a code bitstream obtained from input information having a coding unit time length which is an N integer multiple (N greater than =2) of an error detection unit time length into N bitstreams, calculation means for calculating an error detection code of each divided bitstream output from the division means, and transmission means for sequentially transmitting each divided bitstream output from the division means and the error detection code output from the calculation means in correspondence with each divided bitstream.